Electronic apparatus



Nov. 27, 1945. w. C HAHN ELECTRONIC APPARATUS Filed Nov. 22, 1940 2 Sheets-Sheet l Inventor: William OHahTl, b fi M MMLM y is t torney.

Nov. 27, 1945. w. c. HAHN 2,389,903

ELEGTRONI C APPARATUS Filed Nov. 22, 1940 2 Sheets-Sheet 2 Inventor: WiHiam C.Ha-hn, 5/ x E byflwyflmbzawl His Attorn ey.

Patented Nov. 27, 1945 ELECTRONIC APPARATUS William (2. Hahn, Scotla, N. Y., assignor to General Electric Company, a corporation of New York .7

Application November 22, 1940, Serial No. 366,593

10 Claims.

The present invention relates to electronic apparatus for amplifying or detecting high frequency signals.

It is one object of the invention to provide improved means for converting the variations of a radio signal wave or the like into corresponding variations in an electron stream. It is particularly proposed in this connection to make available a new input electrode system which exhibits negligible losses even when used at extremely high frequencies.

It is a further object of the invention to provide an improved signal amplification or detection system which may be used at ultra-high frequencies and which functions satisfactorily over a relatively wide range of frequencies.

In connection with the attainment of the first of the foregoing objects use is made of an electrode arrangement by which the small velocity variations inherent in an electron stream may be caused to produce in the stream marked variations in electron density determinable in accordance with an input signal. The preferred embodiment of this aspect of the invention utilize a relatively elongated hollow electrode structure which is traversed by the electron stream and which is normally biased to such a potential that it reverses at its central region the lower velocity component of the beam, while permitting passage of the higher velocity components. As will be explained more fully hereinafter, the application of a signal potential to such an electrode servesto' produce charge density variations in the stream Without incurring substantial losses in the electrode structure.

A further aspect of my invention consists in the discovery and application of the fact that charge density variations in an electron stream, (as produced, for example, by the operation of an electrode structure of the type described in the preceding paragraph) tend to become converted, as the stream. progresses, into velocity variations. This type of energy conversion has certain aspects which readily adapt themselves to the production of amplification effect and which are relatively independent of the frequency of the variations dealt with. It is utilized in a practical way by means of a structural combination which includes an electron stream source, means for producing charge density modulation of the electron stream in accordance with an input signal, mean providing a relatively elongated space to be traversed by the charge density modulated stream, and means for causing the velocity variations developed in the stream during its passage through the said space to produce an output current, either for purposes of amplification or detection.

The features which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the drawings in which Fig. 1 is an enlarged longitudinal view of an electronic apparatus embodying one aspect of my invention; Fig. 2 is an enlarged detail of one of the structural elements of the apparatus of Fig. 1; Fig. 3 illustrates diagrammatically another application of the signal input system shown in Fig. 1 and further represents another aspect of my invention; Fig. 4 illustrates a modification of the arrangement of Fig. 3; Fig, 5 shows the application of the invention in connection with a detector; and Fig. 6 is a graphical representation useful in explaining the invention.

Referring particularly to Fig. 1, there is shown a discharge envelope having an elongated shaft portion l0 and a bulbous portion II. The shaft portion It contains at one end thereof a cathode It which is shown as comprising a hollow cylinder having an internal resistance heater indicated in dotted outline at M. The end of the cathode cylinder which is directed toward the main body of the envelope is assumed to be provided with a closure member which is coated with an electronically active material such a alkaline earth oxide. Electrons emitted by the cathode are accelerated by means of an electrode l6 which is maintained positive with respect to the cathode by connection to a battery it. Focusing of the electron stream may be obtained by the use of a magnetic coil positioned as indicated in dotted outline at l9.

In spaced alignment with the electrode it there is provided another electrode 20 which is maintained at the same potential as electrode I5. (Electrode 20 is terminally placed in order to shield the adjacent portion of the envelope from destructive bombardment which tends to occur at this point.) The gap between electrodes l6 and 20 is surrounded by a third electrode 2| which is shown as conforming to the interior wall surface of the envelope. A will be more fully explained in the following, it is the function of this electrode (which may alternatively be inserted directly in the space between electrodes I6 and 20) to produce variations in the electron stream projected from the cathod l3 in accordance with a signal voltage which is desired to be amplified.

The electrode 2| is normally biased to a potential which is below that of the electrodes I6 and 20 and which should be close to that of the cathode |3. The biasing connection may be made, for example, through the combination of a battery 24 and a resistor 25.

The electron stream projected from the cathode |3 will contain electrons of varying velocities as a result of the random velocities with which electrons are emitted from the cathode surface. 'Ordinarily, however, the spread of electron velocities will not be very great and may be equivalent to that produced by voltage differences on the order of a fraction of a volt. Consequently, it is feasible to fix the potential of the electrode 2| (as by adjustment of the biasing resistor 25) to a value such that the retarding field which it produces reverses approximately half the approaching electron beam (i. e. the lower velocity portion of the beam). If the potential of the electrode 2| is varied slightly from the aforesaid value, the relative magnitudes of the reversed and transmitted portions of the beam may be modified accordingly. Moreover, this change may be quite great for slight shifts in the electrode potential, so that a relatively weak signal applied to the electrode may cause relatively great charge den.- sity variations in both the transmitted and reflected parts of the beam. This fact is made use of in accordance with the present invention by applying to the electrode 2| a signal potential which is derived, for instance, from a receiving antenna 28 coupled to the electrode 2| through the tuned combination of a condenser 29 and an inductance 30. In view of the considerations above stated, it will be understood that variations of the potential of the electrode 2| as produced by the input signal will cause corresponding changes in the amount of electron current transmitted through the electrode and will thus render the emerging electron stream charge density modulated in the sense that it becomes characterized by variations in electron density from point to point along its length.

A modulating system such as that described in the foregoing is considered to have special utility in connection with the amplification of high frequency signals in that it offers a possibility of ob-. taining modulation effects without the attendant occurrence of substantial input losses. This fact may best be understood by referring to Fig. 2 gvlhich represents an enlargement of the electrode In the figure just referred to the electron stream entering the electrode 2| is indicated by a pair of parallel rays a and b which represent the components of the beam which are respectively capable and incapable of being reversed by the field existing between the electrode 2| and the preceding electrode l6. If it be assumed that the electron transit times from the point of reversal of the reversed portion of the beam to the points at which the reversed and unreversed electrons respectively leave the electrode 2| are equal, it is clear that the leaving current I) is always equal to the constant entering current (a plus 12) minus the leaving current a. In other words, the net current approaching the entering end of the electrode 2| is always identical with the current leaving the exit end. From still another standpoint, it may be said that in the event of fulfillment of the transit time condition above specified, variations in the leaving current 2) are always accompanied by similar and in-phase variations of the approaching current, a plus 1) minus a.

aaaaooa The foregoing consideration will b recognized as important when it is recalled that the ex-' planation for the occurrence of high frequency losses in an input electrode system of conven tional type lies mainly in the cyclically changing ratio of the charges approaching and leaving the input grid and the effect of this change in inducing currents in the grid circuit. These currents and the energy losses due to them become so great at very high frequencies as to produce substan-' tial inoperability of the devices in which they occur. In connection with the arrangement of Fig. 1, however, currents induced in the grid 2| as a result of variations produced in the leaving current b by thesignal voltage are neutralized, as

far as the external circuit is concerned, by the equal and oppositely directed currents induced by the simultaneous variations in the approaching current (a+b-a). High frequency input losses due to transit time effects therefore become negliible.

Neutralization of the character above described may be secured in one way by disposing the electrode 2| symmetrically in the axial sense with respect to the electrodes l4 and 20, and by maintaining these latter electrodes at the same potential. This is the simplest and therefore the preferred arrangement, but it should be noted that a lack of structural symmetry can be offset to some extent by making the electrodes l4 and 20 of unequal potential in order properly to adjust the electron transit times.

Charge density variations produced by a modulating system such as that described in the foregoing may be used in the production of an output signal the amplitude of which is greatly in excess of that of the input signal. The output voltage may be developed, for example, by collecting the electron stream by an electrode (represented in Fig. 1 as a hollow conical member 33) which is in series with a tuned circuit comprising the parallel combination of a condenser 34 and an inductance 35 and which is coupled to output terminals 36 through this circuit.

The electrode 33 should be maintained at a relatively high potential (as by battery 39) so as to assure the collection by it of all the components of the electron stream which pass the electrode 2|. In order to prevent secondary electrons emitted by the surfaces of the electrode 33 from returning to the modulating space and thus interfering with the intended operation of the apparatus, one may employ a suppressor electrode in the form of a metal cylinder 38. This cylinder is preferably biased to a potential which is several hundred volts negative with respect to the potential of the electrode 33.

Further amplification may be obtained, in accordance with another aspect of my invention, by reconverting the charge density variations produced by an input system such as the electrode 2| of Fig. 1 into velocity variations by the operation of space charge effects This feature of the invention may best be understood by reference to Fig. 3 which represents diagrammatically an electron beam tube comprising a series of tubular electrodes arranged in aligned relation within an envelope 40 (indicated in dotted outline). The electrodes shown include a cathode 4 I, an accelerating electrode 42 and a modulating input system of the type described in Fig. 1. The modulating system comprises the combination of two positively biased electrodes 43 and 44, arranged in spaced relation, and a negatively biased electrode 85 symmetrically interposed between them. The latter electrode is connected to a tuned input circuit formed by an inductance 41 and a condenser t8 and is coupled through this circuit to a receiving antenna 49.

The electrode 45 is biased negatively so as normally to cause partial reversal of the electron stream proceeding from the cathode 4i. As a result, the unreversed or transmitted current may be charge density modulated in accordance with the variations of the input signal, as explained in connection with Fig. 1.

After being thus modulated, the electron stream is next caused to traverse an elongated shielded space provided within the member 44. This shielded space constitutes a relatively field-free region within which the charge density modulation of the beam may be converted into velocity modulation for a purpose to be described at a later point herein. This conversion is efiected by the action of space charge and may be understood by considering the electron beam as analogous to a moving tube of a highly resilient material such as rubber. Referring to this analogue, let it be assumed that forces are applied to the moving tube which set up in it a series of localized longitudinal compressions. At the instant the compressions are established, the molecules of which the tube is comprised may have no relative motion with respect to one another, although all are, of course, moving through space with the velocity of the tube as a whole. An instant later, however, (and after the forces producing compression are removed) the resilience of the tube causes the molecular components of the compressed sections to move away from one another with constantly increasing relative velocities. This process will continue until the potential energy stored in the compressions has been con-' verted into kinetic energy represented by velocity differences among the Various molecular components of the tube.

The foregoing is considered to be the sort of thing which happens in an electron beam which has been subjected to charge density modulation. With the passage of time (that is, with the passage of the modulated beam through space) the mutually repulsive forces of the electrons accumulated at the regions of maximum electron density tend to produce dispersive movements of the electrons in such a way as to create marked velocity variations from point to point along the beam. There is some particular time at which this conversion becomes maximum, that is, at which the velocity modulation of the beam resulting from its initial charge density modulation attains a value which will not be exceeded. This time may be determined by empirical means or may be derived mathematically by taking into consideration the various relevant factors, such as the average beam velocity, the average charge density oi. the beam, etc. In the present case, it is proposed to make the length of the shielding tube 46 sufliciently great so that the average transit time of the electrons through the tube corresponds at least approximately to the time required for the most effective conversion of charge density modulation produced by the electrode 45 into velocity modulation. For the attainment of this purpose, the length of the tube must ordinarily be made at least several times the distance traversed by the electron beam during the period of a single cycle of the signal to which the beam is subjected. In a particular case the optimum from the tubular member 44.

tube length may be computed with fair accuracy by means of the formula:

where L represents the length of the drift space in centimeters;

V0 represents the average velocity of the beam in volts;

Io represents the beam current in milliamperes;

A represents the wave-length of the applied signal in centimeters (in vacuum) a is a constant, the value of which is determined by the dimensions of the envelope and electrode parts. For most practical cases it will fall between 1.0 and 2.0 and may be assigned an average value of 1.3.

As a result of the considerations stated in the foregoing, it may be assumed that the beam as it issues from the tube 44 i characterized by a relatively high degree of velocity modulation, that is to say, by cyclically repetitive variations in electron velocity from point to point along the beam. In order to utilize this factor in the production of useful results, it is necessary to subject the beam to the action of some agency which is responsive to electron velocity differences. As has been previously explained herein, one such means comprises electrode structure adapted to provide a retarding field acting on the beam. A retarding field may be provided in this instance by the use of a tubular electrode 52 which is maintained at a potential at or near that of the cathode M so that it tends to reverse the lower velocity electrons while permitting passage of electrons of higher velocity. An electrode 53 which is maintained at the same potential as the electrode 44 establishes symmetrical electrical conditions around the electrode 52 and thus permits avoidance of high frequency losses in accordance with the principles described in connection with the electrode 2! of Fig. 1.

As a result of the velocity-discriminating action of the electrode 52, the portion of the beam which is transmitted by it is charge density modulated according to a pattern which corresponds to the velocity modulation of the beam as it issues The chargev density modulated stream is caused to impinge on an electrode 54 which is biased to a relatively high positive potential (e. g. by mean of a battery 58) and which is connected in series with a resonant output circuit comprising the combination of a condenser 55 and an inductance 56. The output circuit 55, 56 is coupled to appropriateoutput terminals 57.

The arrangement of Fig. 3 provides a very effective amplification means. In the first place, the action of the electrode 45 enables a relatively weak input signal to produce relatively strong charge density variations in the beam. In addition, the conversion of the charge density variations into velocity modulation by the provision of the shielding tube 64 and the subsequent reconversion into charge density modulation by the action of the retarding electrode 52 results in the production of very marked amplification efiects. The amount of this latter amplification is measured by the quotient of the factor which determines the effectiveness of the electrode 52 in converting velocity modulation into charge density modulation by the factor which measures the ratio between the charge density modulation of a beam entering the tube and the velocity modulation of the same beam leaving it. The first-named factor may be on the order of 500 t 1,000 while the second may be on the order oi about 50. Consequently. an over all amplification of as much as or 20 to 1 may be produced in a single stage of an amplifier such as that.- described.

It is a further noteworthy feature of the amplification system described in the foregoing that no resonant circuits are required in connection with the various intermediate stages of amplification (e. g. the stage provided by the electrodes 44, 52 and 53 of Fig. 3) Consequently, the operating band width of the system as a whole may be relatively great, and in this respect the system described possesses a considerable advantage over amplifying arrangements previously proposed for high frequency use.

The energy conversion system described in the foregoing may be alternatively employed in connection with a difierent type of input electrode system, this alternative use being illustrated in Fig. 4. In this case, the electronic apparatus includes an envelope 60 (indicated in dotted outline), a cathode 6|, an accelerating electrode 62, a series of tubular elements which are numbered 64 to 10, inclusive, and a collecting electrode II.

The electrodes 64, 65 and 66 constitute a modulating system of the type described and claimed in mycopending application, S. N. 153,602, filed July 14, 1937, now Patent No. 2,220,839, November 5, 1940. All three of the electrode elements referred to are maintained at a positive potential with respect to cath'ode 8| by means of a battery 13. It is thefunction of the members 64 and 66 to define a relatively shielded space within which modulating action may occur. Input potential is applied to the electrode 65 through a tuned circuit which includes the combination of an in-- ductance I5 and a condenser 16 and which is coupled to a receiving antenna Tl.

The axial length of the member 65 is made such that the average electron transit time through the electrode corresponds at least approximately to a half cycle of the potential variation of the input signal or to some odd number of such half cycles. Under these circumstances, electrons which enter the modulating space in such time phase asto be accelerated when they traverse the gap between the members 64 and 65 (i. e, when the potential level of the electrode 15 is relatively higher than that of the electrodes I4 and 15) are again accelerated as they leave the electrode member 15, that is, as they traverse the gap between the members 65 and 66. Conversely, electrons which reach the modulating space one half cycle later, that is, when the potential level or the member 65 is relatively low, are twice decelerated as they successively traverse the interelectrode gaps. As a result of the modulating action thus produced, the electron stream issuing from the modulating space is necessarily velocity modulated in the sense that it exhibits recurrent variations in electron velocity from point to point along its length. Moreover, for reasons which are stated fully in my aforesaid Patent No. 2,220,839, this modulation is accomplished without the occurrence of substantial input losses.

The velocity modulation produced by the electrode 65 may be converted into charge density modultion of a higher order of magnitude in accordance with principles previously described herein by causing the modulated streams to traverse a retarding field which is eflective to reverse the lower velocity components of the beam. Such a field may be provided, for example, by applying to the tubular member 91 a relatively negative voltage.

As the next step in the amplification process. the charge density modulation developed as above specified is reconverted into velocity modulation by passing the modulated beam through the relatively field-free space provided within the tube 69. The velocity modulation existing in the beam as it issues from this space may be'converted into an output current with attendant amplification eflects by means of electrodes-99, I0 and H, the first or these being maintained at a negative potential adapted to provide a retarding field and the other two being at a relatively positive potential. The electrode H, which is a collect ing electrode, is connected to an outputicircuit which includes tuned reactive elements 99 and ill and which is coupled to output terminals 9!. It will be recognized that the electrodes 69, I9 and II correspond in nature and function to the electrodes 52, 53 and 54 of Fig. 3.

The system just described comprises in efiect a multistage amplifier in which the first stage of amplification involves the action of the retarding field of the electrode 91 on the electron beam (after modulation thereof by the electrode 65), and the combination of the members 89 and 69 provides a further stage of amplification. Obviously, still further stages of the character provided by the members 68 and 69 may be added in cascade arrangement if desired.

Fig. 5 represents a further application of the invention to a detector. Here, in addition to an envelope 90 (shown in dotted outline), a cathode 9| and an accelerating electrode 92, there are provided a series of aligned tubular members 94, and 96 and a collecting electrode 91. The electrodes 94 and 98 are maintained at relatively positive potentials (e. g. by a battery 99) while the electrode 95 is relatively negative. The input signal, as derived from a receiving antenna I99, is applied to the electrode 95 through a tuned circuit comprising the parallel combination of an inductance NH and a condenser I02. The resultant variations in potential level of the electrode 95 produce charge density modulation of the affected portion of the beam in the manner previously described in connection with Fig. 1.

The charge density variations existing in the beam as it issues from the electrode 95 are reconverted into velocity modulation by passing the beam through a shielded space provided within the electrode 98. In order to produce detection effects, the electrode 91 is biased negatively with respect to the electrode 96 (as by appropri-- ate connection to the battery 99) so as to provide a retarding field of such character that the ratio of the reversed and collected components of the beam varies non-linearly with the electron velocity. This may be better understood by referring to Fig. 6 which represents graphically the static characteristic of the device of Fig. 5. In this figure, the curve A illustrates the variation in electron current (Ip) received by the electrode 91 as the potential (E 01' this electrode is varied from a strongly negative to a less negative value. For an appreciable portion of its extent, this curve is approximately linear and is quite steep in slope. Consequently, by operating on this part of the curve, say, at :c, marked amplification effects may be obtained in the manner described in connection with Fig. 3.

in series with the electrode 91 and to which output terminals lot are connected.

While the invention has been described by ref erence to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, means for producing a stream of charged particles movin along a given axis, such means including a particle source and an electrode for accelerating particles from the source along the given axis, an electrode structure arranged to be traversed by the said stream and having appreciable extension in the direction of the said axis, means for biasing the said electrode structure to a potential which is below that of the said accelerating electrode and at which it causes reversal of a material portion of the beam at a point near the central transverse plane of the structure, means for cyclically varying the potential level of the electrode structure at a signal frequency in order correspondingly to vary the relative magnitudes of the reversed and unreversed portions of the stream, and means for utilizing variations which exist in the stream subsequent to the action of the said electrode structure thereon.

2. In combination, means including a cathode and an accelerating element for producing a beam of electrons moving along a given axis, a tubular electrode surrounding the beam path and having appreciable extension in the direction of the said axis, means for biasing the said electrode to a potential which is close to that of the cathode and at which it causes reversal of a material portion of the beam to occur at a point near the central transverse plane of the electrode, means for cyclically varying the potential level of the electrode at a signal frequency in order to vary the relative magnitudes of the reversed and unreversed portions of the beam, and means for utilizing the variations which exist in the beam subsequent to the action of the said electrode thereof.

3. In combination, means for producing a beam of electrons moving along a given axis, an electrode system including a, pair of spaced conductive elements maintained at a positive potential with respect to the source of the beam and an electrode interposed between the said conductive parts and having appreciable extension in the direction of the said axis, means for biasing the said electrode to a potential which is negative with respect to the potential of the said conductive parts and at which it causes reversal of a material portion of the beam to occur at a point near the central transverse plane of the electrode, means for cyclically varying the potential level of the said electrode at a signal frequency in order to vary the relative magnitudes of the reversed and unreversed portions of the beam, and means for utilizing the variations which exist in the beam subsequent to its issuance from the said electrode system.

4. In combination, means for producing a stream of electrons, means acting on the stream to produce charge density variations therein, means providing a shielded space to be traversed by the steam subsequent to the operation of the last-named means thereon, and in which the charge density variations may be converted into electron velocity variations as a. result of space charge effects, and means for utilizing the velocity variations existing in the stream after its passage through the said shielded space.

5. In combination, means for producing a stream of electrons, means acting on the stream to produce charge density variations therein, means providing a space to be traversed by the beam subsequent to the operation of the lastnamed means thereon, said space being sufilciently long to permit the efiective conversion of the charge density variations in the stream into electron velocity variations as a result of space charge efiects, and means for utilizing the velocity variations existing in the stream after its traversal of the said space.

6. In combination, means for producing a stream of electrons, means acting on the stream to produce charge density variations therein, means providing an elongated space to be traversed by the stream subsequent to the operation or the last-named means thereof, said space being or sufiicient length to permit the effective conversion of the charge density variations of the stream into electron velocity variations, and means providing a retarding field acting on the beam subsequent to its traversal of the said space for utilizing the said velocity variations in the production of an output current.

7. In combination, means for producing a stream of electrons, means acting on the stream to produce charge density variations therein, means providing an elongated space to be traversed by the stream subsequent to the operation of the ,last-named means thereon, said space being of suflicient length to permit the effective conversion 01' the charge density variations of the stream into electron velocity variations, and means for utilizing the said velocity variations in the production of an output current, said lastnamed means including electrode structure for producing a retarding field acting on the stream subsequent to its traversal of the said space and adapted to produce at least partial reversal of the stream.

8. An amplification system including means for producing a stream of electrons, means acting on the stream to produce charge density variations therein, means providing a shielded space to be traversed by the stream subsequent to the operation of the last-named means thereof and in which the charge density variations may be converted into electron velocity variations, and means for reconverting the said velocity variations into charge density variations of a. higher order of magnitude.

9. The method of amplifying high frequency impulses which comprises producing a stream of charged particles, operating on the stream to produce charge density variations therein, directing the stream along a relatively long path to cause conversion of the charge density variations into velocity variations as the result of space charge efiects, thereafter subjecting the stream to the action of a retarding field to produce an output current which is variable by virtue of, and tion into charge density modulation by the ap in accordance with, the variations in the action plicatlon of a retarding field, and thereafter diof the said retarding field on the particles of recting the stream along a path of sufficient diflferent velocity. length to cause the reconversion oi. the charge 10. The method of amplifying the velocity 5 density modulation into velocity modulation as variations of a velocity modulated electron stream a result of space charge effects.

which comprises converting the velocity modula- WILLIAM C. HAHN. 

